Eternal punishment is pointless.

[Nope]

How is it possible that Jewish people read the Old Testament and do not believe in the concept of original sin?

I don't know how rabbis actually interpret this aspect of Genesis but think that while religious people believe in their scriptures they try to some extent to avoid moments that are uncomfortable to them such as contradictions or extremities (what they subjectively treat as unjustice, extreme violence or moral burden). Mild believers try to avoid both contradictions and extremities while fundamentalists try to avoid mostly contradictions (as extremities don't pose that much threat to their faith). So as soon as the concept of Original Sin isn't so specifically outlined (thus is easier to avoid) in Judaism (correct me if I'm wrong) it's not surprising that people don't make a deal out of it. Also as far as I know Original Sin is used profoundly in anti-semitic rhetorics so this may actually be the main reason why it isn't given spotlight I guess.

When you read the Old Testament in context without the influence of Christianity, there really isn't anything that indicates there is original sin. Sure biblical people did bad things and talk about how much greater god is then they are but there isn't any real concept of original sin.

From the same article that I quoted in my other post.

While in Christian theology Job’s personal spiritual triumph is a theological impossibility, in Jewish terms it stands out as the embodiment of God’s salvation program for mankind. Job didn’t rely on Jesus to save him and he certainly did not turn to the cross for his redemption; rather, it was his obedience to God that made his life a paradigm for all humanity.

If such a character as Jesus existed, he wouldn't have understood original sin because he was Jewish. The religion Christianity made up the idea of original sin to explain why a savior was needed.